One time programmable (OTP) memory generally represents a class of memory that, during normal use, may be programmed only once. Although different techniques have been used in the past to implement OTP memory arrays, the techniques typically rely on information being stored on the insulated gate of a metal oxide semiconductor (MOS) transistor. As deep sub-micron technologies featuring very thin gate oxides become more prevalent, however, these implementations tend to result in high gate leakage current preventing long retention time of the information.
OTP memory arrays often utilize polysilicon fuse elements that are programmed by allowing a high current to flow through the fuse elements causing the fuse element to “thermally blow”. Application of such high voltage and high current typically required to thermally blow a polysilicon fuse, however, may cause other circuit elements to be damaged. Accordingly, many such OTP memories require large protection devices that are designed to carry the programming current (typically around 100 mA), while at the same time isolating current sensitive devices from the large programming current. Unfortunately, the use of these protection devices typically causes such fuse elements to occupy a very large silicon area (typically greater than 1000 μm2).
Additionally, the blowing of a polysilicon fuse is an unreliable process that does not act cumulatively. That is, if a fuse does not blow correctly upon application of a programming current, it cannot be further blown by applying the programming current again. Consequently, a certain level of redundancy is typically required to compensate for the small number of fuse elements that do not blow correctly on the first try. However, this redundancy requires further amounts of the scarce silicon area.